Considerable evidence now indicates that mRNA in eukaryotic cells is derived from large DNA-like heterogenous nuclear RNA (HnRNA). Two post-transcriptional modifications are known to be involved in the production of mRNA from HnRNA. These are the addition of polyadenylic acid (poly(A)) sequences to the 3'-terminal of HnRNA and the cleavage or processing of HnRNA to yield mRNA. Since most of the mRNA in eukaryotic cells apparently contain poly(A), these events define the major pathway for generating mRNA. Further details of the events in the biogenesis of mRNA in eukaryotic cells have not been established. Recently, it was shown that both the mRNA for immunoglobulin heavy chain protein and larger nuclear precursor RNA containing the heavy chain coding sequences can readily be purified from immunoglobulin producing myeloma cells. This specific mRNA system is being employed to examine the role of poly(A) sequences and the nature of the post-transcriptional processing events in the biogenesis of mRNA from HnRNA.